1. Field of the Invention
The invention is a pyroelectric thermal detector.
Radiation detectors fall into two broad categories, based on the nature of the energy detected. Quantum, or photon, detectors respond to the effects of discrete electronic excitations within the detector material caused by the action of the individual photons that form the beam of radiation. Thermal detectors, on the other hand, respond to changes in the temperature of the detector material caused by absorption of energy from the incident beam. This absorption of the radiant energy may be accomplished either directly by adsorption within the detector material itself, or indirectly by absorption in a suitable auxiliary structure that conducts the heat to the detector material.
The different types of thermal detectors are characterized by the particular thermally varied characteristic of the detecting material that is monitored in each case. There are thermal detectors that depend on the thermal expansion of solids, liquids or gases; on variations in electrical resistance (bolometers); in thermoelectric power(thermocouples); in dielectric permittivity (dielectric bolometers); in the resonant frequency of piezoelectric crystal resonators; in the spontaneous magnetization of ferromagnetic materials (pyromagnetic detectors); and in the spontaneous electrical polarization of pyroelectric materials (pyroelectric detectors).
2. Prior Art
Pyroelectric thermal detectors operate at normal ambient temperatures and this makes them desirable for use as infra red (IR) detectors in comparison with detectors that require cooling. In a typical pyroelectric detector, incident radiant heat flux is absorbed by the pyroelectric crystal and is converted into heat. The change in crystal temperature resulting from the heat absorption alters the lattice spaces within the crystal and produces a change in spontaneous electric polarization. Electrodes are placed on the crystal surfaces that are normal to the crystal polarization axis. When the electrodes are connected to an external circuit, a current proportional to the rate of change of temperature is generated that will, for instance, produce a voltage change across a load resistor.
The sensitivity of a detector is commonly expressed in terms of the noise equivalent power (NEP) which is defined as the incident signal power, normalized to unit frequency bandwidth at a given operating frequency and radiation wavelength, for which the signal-to-noise ratio is unity. Another common measure of detector sensitivity is the detectivity (D*) given by ##EQU1## where R.sub.v = voltage responsivity (in Volts/Watt),
A = area of the detector element (in cm.sup.2), and PA1 V.sub.N = voltage noise of the detector-preamp combination referred to the preamp output [in Volts/Hz).sup.1/2 ].
An increase in D* can be obtained by maximizing the responsivity R.sub.v and/or minimizing the noise voltage V.sub.N.
Noise voltage V.sub.N is produced by both intrinsic noise sources, located within the detector or its preamplifier, and ambient noise sources. Noise voltages due to ambient noise sources may be reduced either directly, by attenuation of the source, or indirectly, by circuitry designed to minimize the voltage generated in the detector. One type of such circuitry cancels noise voltage while retaining the signal voltage. Two complementary detecting elements are used, only one of which is exposed to the signal source at any time, but both of which produce identical noise voltages generated by ambient noise sources. The two detecting elements are connected in opposition so that the noise voltages cancel, while the signal voltage, appearing on only one element at any given time, is not cancelled. Such circuitry is shown in U.S. Pat. No. 3,453,432 to McHenry and in U.S. Pat. No. 3,842,276 to Southgate. Prior art devices or systems for reduction of ambient noise in pyroelectric detectors have focused on noise generated by ambient temperature variations. Ambient noise in pyroelectric detectors, however, is generated not only by temperature variations, but also by ambient vibrations, since the piezoelectric properties of pyroelectric materials cause mechanical vibrations to be translated into voltages. Vibration induced noise can be significant in a detector to be used, for instance, in an aircraft or missile. Cancellation circuitry ambient noise reduction techniques require as nearly identical noise voltages as possible on each complementary detecting element, and the prior art has not, to our knowledge, addressed the problem with regard to vibration noise.
Further, our detector is to have a high D* and be suitable for operation at low modulation frequencies, at or below about 1000 Hz, both requirements posing additional difficulties in designing a detector with low susceptibility to vibration. Since most intrinsic noise sources are inverse functions of the thickness of the pyroelectric detecting wafer, and since a high D* requires low noise, both intrinsic and ambient, then elements are necessary. The design of detectors for operation at modulation frequencies of 1000 Hz or below imposes special problems, since to achieve maximum D* the active detecting region, that is, the portion of the pyroelectric wafer located between the electrodes, must be thermally isolated. This requirement precludes the conventional construction technique of mounting the detecting wafer flat on a substrate, which would minimize the effects of ambient vibration on the detecting wafer. A low frequency detector having a high D*, therefore, means a thin wafer with a thermally isolated detecting element, a difficult device in which to reduce noise voltage due to ambient vibration.